Clinical implications of anatomical wear characteristics in slipped capital femoral epiphysis and primary osteoarthritis.

BACKGROUND This study compares the wear characteristics in slipped capital femoral epiphysis (SCFE) with those of primary osteoarthritis (OA) in adult patients with advanced arthritis. METHODS One hundred femoral heads and proximal neck specimens were studied from SCFE patients (16 hips) and from primary OA (84 hips) patients undergoing total hip arthroplasties (THA). Grade 4 chondromalacia was plotted on a 2-dimensional (2-D) paper grid. Computer tomographic scans were used to create 3-D models of the femoral head and neck to trace the wear patterns. RESULTS The SCFE group was characterized by (1) loss of neck-head offset, (2) acetabular neck impingement, and (3) loss of superior peripheral articular cartilage adjacent to superior neck. Whereas the primary OA group showed (1) preservation of head-neck offset, (2) absence of acetabular neck impingement, and (3) preservation of superior peripheral articular cartilage. The 3-D modeling in SCFE specimens demonstrated acetabular impingement on the superior lateral femoral neck causing the femur to externally rotate with flexion. The SCFE patients undergoing THA on average were 11 years younger than those with primary OA. The study strongly suggests that the abnormal rotation of the femoral head in SCFE patients causes thinner superior lateral articular cartilage on the femoral head to articulate with the acetabulum. The pistol-grip deformity of the proximal femur in the SCFE group results in hip impingement, which explains why hip flexion and internal rotation can be restricted. CONCLUSIONS There was a premature development of advanced OA of the adult hip joint in SCFE patients. This was associated with hip impingement caused by loss of the head-neck offset and reorientation of the articular cartilage of the femoral head. Unless the femoral head is redirected in patients with SCFE, the benefits of limited hip preservation debridement procedures are not expected to delay the onset and progression of arthritis. LEVEL OF EVIDENCE Prognostic study.